1. FIELD OF THE INVENTION
The present invention relates to an inverter, and more particularly to an inverter for use with induction motors of which logic is composed solely of software embedded in a microprocessor.
2. DESCRIPTION OF THE PRIOR ART
Prior art inverter logics such as those disclosed in U.S. Pat. No. 4,259,845, entitled, "Logic Control System for Inverter Driven Motor," granted to Norbeck are composed of analog hardware including a voltage controlled oscillator (VCO), comparators and analog-to-digital (A/D) converters and the like. Those inverters typically use thyristors for switching the line voltage power source and inverter output power. Hence, the functional capability is limited by the hardware arrangement of which structure is generally complex and thus expensive. Prior art inverter's voltage/frequency (v/f) characteristics are given a priori or are at most selectable manually. They are usually preprogrammed by hardwires to give either a constant torque, a constant power, or a combination thereof. Therefore, an optimum primary voltage cannot be selected as a function of instantaneous torque load demand, resulting in inefficient operation of an induction motor coupled to the inverter. To overcome this inefficiency problem, there is a teaching in U.S. Pat. No. 4,217,763 entitled, "Asynchronous Motor-Driven Machine having Variable Torque Demand such as Household Refrigerator Compressors," granted to Hammele et al. that the asynchronous motor is driven by a phase-angle control device at an optimal operating voltage matched to required torque to yield substantially optimal efficiency. However, within the scope of this patent, the teaching is limited only to substantially constant speed motors. Recently, there appeared some inverters which are composed of a digital logic stage and a transistor power stage. They are generally characterized by pulse width modulation (PWM) of a sine wave since such a modulation is possible by the transistor power stage with a faster switching rate compared to thyristors.
When PWM is employed to simulate sine functions of a continuous wave, the carrier frequency generally varies as a function of the desired inverter output frequency. This results in lower carrier frequencies at lower inverter output frequencies which leads to degraded efficiencies and increased noise and vibration levels of the motor.
The bit patterns associated with PWM are generally stored in a read-only-memory (ROM), because the clock period is too short to calculate a sine function on a realtime basis. As a consequence, a relatively large-scale ROM is need to memorize bit patterns for all possible sets of frequency/primary voltage. Moreover, as 0 and 1 are typically memorized in ROM's cells as is, a relatively large number of bits are necessary.
Inverters switch their frequency/primary voltage on a realtime basis. They therefore tend to lose the synchronization between the U-, V- and W-phases due to the time delay in switching, which sometimes leads to a motor stall.